The present invention relates to a method for treating odorous gases of a chemical pulp mill and for improving the control of nitrogen oxide emissions.
In sulfate pulping, wood is treated in white liquor containing sodium hydroxide and sodium sulfide, whereby the lignin is hydrolyzed. Thereby several organic sulfur compounds are formed, such as methylmerkaptan, dimethylsulfide and dimethyldisulfide. These very compounds together with hydrogen sulfide cause the unpleasant smell of exhaust gases of chemical pulp mills. These gases are formed in several stages of a chemical pulping process, such as at the digester plant and the waste liquor evaporation. Malodorous sulfur compounds are removed most usually by collecting the malodorous gases from various sources and by combusting them either in a lime kiln, a chemical recovery boiler or a separate combustion apparatus. During combustion all sulfur-containing substances are oxidized to sulfur dioxide, sulfur trioxide, and, in the presence of alkali, also to sodium sulfate, and they are passed into flue gases.
In addition to sulfur compounds, digestion generates also methanol and ammonia. Vapors containing sulfur compounds, ammonia and methanol are released abundantly for instance in black liquor evaporation, where said compounds are distilled and condensed into condensates of a multistage evaporation plant. Foul condensates are usually purified in a steam stripper, where the condensate and steam are put into contact with each other and impurities are transferred from the condensate into the steam, while the condensate stream is obtained in purified form for further use. The exhaust vapor from the stripper is led via a post-condenser to combustion or directly to methanol liquefaction. Non-condensable gases (NCG) are combusted together with the flow of other odorous gases of the mill.
The odorous gases are typically divided into strong odor gases (LVHC Low Volume High Concentration) and dilute odorous gases (HVLC, High Volume Low Concentration). The strong odorous gases originate mainly from the digester plant, the evaporation plant or stripping. Dilute odorous gases are collected from containers and devices from the fiber line, evaporation plant, tall oil plant and causticizing plant. Dilute odorous gases contain the same components as the strong odorous gases, but they also contain so much air that the concentrations are remarkably lower.
The purpose of odorous gas combustion is to oxidize the reduced sulfur compounds contained in the gas, such as hydrogen sulfide, sulfur dioxide, and therefore the combustion is to take place in the presence of a remarkable volume of excess air (e.g. approximately 3-4%) and at a high temperature. Thereby the ammonia contained in the odorous gas is in its turn oxidized into nitrogen oxides. Especially the strong odorous gases contain nitrogen compounds, so that their combustion specifically has an influence on the nitrogen oxide emissions of the mill.
Finnish patent publication 105215 discloses a method, in which ammonia is removed from odorous gases prior to their combustion, whereby the nitrogen oxide content of the flue gas generated in the combustion can be significantly reduced. Preferably the ammonia is removed by scrubbing said gases in order to bind the ammonia off them. The scrubbing solution can preferably be a bisulfite solution originating from the scrubbing of flue gases formed in the combustion of the gases. Some other applicable solution originating from the chemical pulp mill and having a pH in the neutral or acid range, such as acid bleaching effluent or waste acid from the chlorine dioxide plant can also be used.